Numerous retractable ballpoint pens which house in a barrel a ballpoint pen refill in which a water based ink for the ballpoint pen is filled in an ink reservoir and a back end of a portion filled with the ink is provided with a backflow-preventing member (ink follower) and which requires no cap have been marketed. In the retractable ballpoint pen, when the housed ink has a relatively high viscosity, ink drooling from the pen tip can be prevented to some extent, but when ink having a low viscosity is used, the ink drooling easily occurs.
The drying resistance of the water based ink for the ballpoint pen is important when the pen tip is exposed. Thus, it has been attempted to increase the amounts of urea and a derivative thereof as an anti-drying agent for the pen tip (e.g., see Patent Document 1).
However, urea and the derivative thereof exhibit the drying resistance by virtue of their high moisture absorption capacity. Thus, when the ink excessively containing these is left to stand under a condition at high humidity, moisture is absorbed locally at the pen tip. Therefore, the viscosity of the ink in a pen tip portion is reduced due to the moisture absorption, and phenomena such as “ink drooling” where the ink is pooled in the pen tip portion and “ink dropping” where the pooled ink is dropped easily occur. In particular, among coloring agents, the ink using a pigment hardly causes the ink drooling and the ink dropping because the pigment is dispersed in a particulate state in the ink, while in the case of using a dye, it is difficult to control the ink drooling and the ink dropping particularly in the ink having a low viscosity.
As described above, although means exist for enhancing the dry up resistance of the ink composition, other writing performance is not sufficiently satisfied, such as the ink drooling and the ink dropping that will likely occur. In particular, when the ink composition is used for the retractable ballpoint pen, it is an important requirement not to produce the ink drooling and the ink dropping simultaneously together with the dry up resistant performance.
Patent Document 1: Japanese Published Patent Application No. Sho-59-45372-A